Flexography is a method of printing that is commonly used for high-volume runs. Flexography is employed for printing on a variety of substrates such as paper, paperboard stock, corrugated board, films, foils and laminates. Newspapers and grocery bags are prominent examples. Coarse surfaces and stretch films can be economically printed only by means of flexography. Flexographic printing plates are relief plates with image elements raised above open areas. Generally, the plate is somewhat soft, and flexible enough to wrap around a printing cylinder, and durable enough to print over a million copies. Such plates offer a number of advantages to the printer, based chiefly on their durability and the ease with which they can be made.
In flexographic printing, ink is transferred from a pool of ink to a substrate by way of a printing plate. The surface of the plate is shaped so that the image to be printed appears in relief, in the same way that rubber stamps are cut so as to have the printed image appear in relief on the surface of the rubber. Typically, the plate is mounted on a cylinder, and the cylinder rotates at a high speed such that the raised surface of the printing plate contacts a pool of ink, is slightly wetted by the ink, then exits the ink pool and contacts a substrate material, thereby transferring ink from the raised surface of the plate to the substrate material to form a printed substrate. Those involved in the flexographic printing industry are constantly striving to improve the flexographic printing process in order to more effectively compete.
Flexographic printing plates are relief plates with image elements raised above open areas. Such plates offer a number of advantages to the printer, based chiefly on their durability and the ease with which they can be made. A typical flexographic printing plate as delivered by its manufacturer is a multilayered article made of, in order, a backing or support layer; one or more unexposed photocurable layers; optionally a protective layer or slip film; and often, a protective cover sheet.
The backing layer lends support to the plate and can be formed from a transparent or opaque material such as paper, cellulose film, plastic, or metal. Preferred materials include sheets made from synthetic polymeric materials such as polyesters, polystyrene, polyolefins, polyamides, and the like. One widely used support layer is a flexible film of polyethylene terephthalate.
The photopolymer layer(s) can include any of the known binders (oligomers), monomers, initiators, reactive or non-reactive diluents, fillers, and dyes. The term “photocurable” or “photopolymer” refers to a composition which undergoes polymerization, cross-linking, or any other curing or hardening reaction in response to actinic radiation with the result that the unexposed portions of the material can be selectively separated and removed from the exposed (cured) portions to form a three-dimensional or relief pattern of cured material. Preferred photopolymer materials include an elastomeric compound (binder), an ethylenically unsaturated compound having at least one terminal ethylene group, and a photoinitiator. More than one photocurable layer may also be used. Examples of some suitable photocurable materials are described in European Patent Application Nos. 0 456 336 A2 and 0 640 878 A1 to Goss, et al., British Patent No. 1,366,769, U.S. Pat. No. 5,223,375 to Berrier, et al., U.S. Pat. No. 3,867,153 to MacLahan, U.S. Pat. No. 4,264,705 to Allen, U.S. Pat. Nos. 4,323,636, 4,323,637, 4,369,246, and 4,423,135 all to Chen, et al., U.S. Pat. No. 3,265,765 to Holden, et al., U.S. Pat. No. 4,320,188 to Heinz, et al., U.S. Pat. No. 4,427,759 to Gruetzmacher, et al., U.S. Pat. No. 4,622,088 to Min, and U.S. Pat. No. 5,135,827 to Bohm, et al., the subject matter of each of which is herein incorporated by reference in its entirety.
The photopolymer materials generally cross-link (cure) and harden through radical polymerization in at least some actinic wavelength region. As used herein, actinic radiation is radiation capable of effecting a chemical change in an exposed moiety. Actinic radiation includes, for example, amplified (e.g., laser) and non-amplified light, particularly in the UV and violet wavelength regions.
The slip film is a thin layer, which generally rests upon and protects the photopolymer from dust and increases its ease of handling. In a conventional (“analog”) plate making process, the slip film is transparent to UV light. The printer peels the cover sheet off the printing plate blank, and places a negative on top of the slip film layer. The plate and negative are then subjected to flood-exposure by UV light through the negative. The areas exposed to the light cure, or harden, and the unexposed areas are removed (developed) to create the relief image on the printing plate. Instead of a slip film, a matte layer may also be used to improve the ease of plate handling. The matte layer typically comprises fine particles (silica or similar) suspended in an aqueous binder solution. The matte layer is coated onto the photopolymer layer and then allowed to air dry. A negative is then placed on the matte layer for subsequent UV-flood exposure of the photocurable layer. In another alternative, the negative may be placed directly on the at least one photocurable layer.
In a “digital” or “direct to plate” plate making process, a laser is guided by an image stored in an electronic data file and is used to create an in situ negative in a digital (i.e., laser ablatable) masking layer. The digital masking layer is typically a slip film which has been modified to include a radiation opaque material. Portions of the laser ablatable layer are ablated by exposing the masking layer to laser radiation at a selected wavelength and power of the laser. Examples of laser ablatable layers are disclosed for example, in U.S. Pat. No. 5,925,500 to Yang, et al., and U.S. Pat. Nos. 5,262,275 and 6,238,837 to Fan, the subject matter of each of which is herein incorporated by reference in its entirety. Other methods of creating an in situ negative are also known.
After imaging, the photosensitive printing element is developed to remove the unpolymerized portions of the layer of photopolymer material and reveal the crosslinked relief image in the cured photosensitive printing element. Typical methods of development include washing with various solvents or water, often with a brush. Other possibilities for development include the use of an air knife or heat plus a blotter (i.e. thermal development). The resulting surface has a relief pattern that reproduces the image to be printed. The relief pattern typically comprises a plurality of dots, and the shape of the dots and the depth of the relief, among other factors, affect the quality of the printed image. After the relief image is developed, the relief image printing element may be mounted on a printing press and printing commenced.
Photocurable resin compositions typically cure through radical polymerization, upon exposure to actinic radiation. However, the curing reaction can be inhibited by molecular oxygen, which is typically dissolved in the resin compositions, because the oxygen functions as a radical scavenger. Based thereon, the dissolved oxygen can be removed from the resin composition before image-wise exposure so that the photocurable resin composition can be more rapidly and uniformly cured.
One process that has been developed for removing dissolved oxygen from the photosensitive resin involves the use of a laminated oxygen barrier layer, as described, for example, in U.S. Pat. Pub. No. 2014.0004466 to Vest et al., the subject matter of which is herein incorporated by reference in its entirety. The barrier membrane is laminated on top of the flexo plate to cover the in situ mask and any uncovered portions of photocurable layer. The membrane can be applied after the laser ablation used to create the in situ mask, but before exposure to actinic radiation.
Effective barrier layers generally exhibit optical transparency, low thickness and oxygen transport inhibition. Examples of materials which are suitable for use as the barrier membrane layer include those materials that are conventionally used as a release layer in flexographic printing elements, such as polyamides, polyvinyl alcohol, hydroxyalkyl cellulose, copolymers of ethylene and vinyl acetate, amphoteric interpolymers, cellulose acetate butyrate, alkyl cellulose, butryal, cyclic rubbers, and combinations of one or more of the foregoing. In addition, films such as polypropylene, polyethylene, polyvinyl chloride, polyester and similar clear films can also serve well as barrier films.
A typical process for producing a relief image printing element from an analog printing element generally involves the customer performing the following steps:                a) obtaining an unexposed printing element from the manufacturer, the unexposed printing element comprising one or more photocurable layers disposed on a substrate layer, a slip film layer disposed on the one or more photocurable layers and a protective coversheet;        b) removing the protective coversheet from the unexposed printing element and applying a photographic negative on top of the slip film layer;        c) optionally, laminating an oxygen barrier layer to the top of the photographic negative;        d) exposing the one or more photocurable layers to actinic radiation through the photographic negative to selectively crosslink and cure portions of the one or more photocurable layers, wherein the at least one photocurable layer is crosslinked and cured in the areas that are not covered by the negative, thereby creating the desired relief image;        e) removing the negative (and the oxygen barrier membrane, if used) from the top of the at least one photocurable layer; and        f) developing the printing blank to remove uncured portions of the photocurable layer and reveal the desired relief image;        
A typical process for producing a relief image printing element from a digital printing element generally involves the customer performing the following steps:                a) obtaining an unexposed digital printing element from the manufacturer, the unexposed printing element comprising one or more photocurable layers disposed on a substrate layer, a laser ablatable layer disposed on the one or more photocurable layers and a protective coversheet;        b) imaging the at least one photocurable layer by selectively ablating the laser ablatable mask layer to create an image on the surface of the at least one photocurable layer;        c) optionally, laminating an oxygen barrier membrane to a top of the laser ablated mask layer;        d) exposing the printing blank to actinic radiation through the oxygen barrier membrane and laser ablated mask layer (and the oxygen barrier membrane, if used) to one or more sources of actinic radiation to selectively crosslink and cure portions of the at least one photocurable layer, wherein the at least one photocurable layer is crosslinked and cured in the portions not covered by the mask layer, thereby creating the relief pattern;        e) removing the oxygen barrier membrane (if used) from the top of the laser ablated mask layer; and        f) developing the printing blank remove the laser ablated mask layer and uncured portions of the photocurable layer and reveal the relief pattern.        
In both instances, the customer must choose a pre-configured printing element blank from a menu of printing element blanks offered by the manufacturer. The customer does not have any control over the particular configuration of the printing element. Thus, the selected printing element may not be the most suitable for the customer's printing requirements. Furthermore, the manufacturer must also maintain an extensive menu of printing element configurations to offer to customers. These configurations include, for example, digital and analog plate configurations, which may have different photocurable layers with different formulations, with and without cap layers, and with and without other specialized layers. This can lead to the need for additional storage space on the part of both the manufacturer and the customer for the many different products. In addition, there is also the additional expense and associated waste in connection therewith if a customer must go through trial and error to select a particular printing plate configuration that might or might not be suitable for their particular needs.
Thus, it would be desirable in the art to provide a method of creating a custom relief image printing blank that would allow a customer to configure a printing plate according to their particular needs and that would allow a manufacturer to offer various raw materials for such configuration.